Rhodium plating composition and method for plating rhodium

ABSTRACT

AN AQUEOUS ACID BATH FORPRODUCING BRIGHT RHODIUM ELECTRODEPOSITS HAVING LOW INTERNAL STRESS CONTAINS RHODIUM ION, ALUMINUM ION SLFATE RADICAL AND A POLYCARBOXYLIC ORGANIC ACID. PREFERABLY, IT ALSO CONTAINS A SMALL AMOUNT OF THE PHOSPHATE RADICAL.

Int. Cl. C231) 5/24 US. Cl. 204-47 7 Claims ABSTRACT OF THE DISCLOSURE An aqueous acid bath for producing bright rhodium electrodeposits having low internal stress contains rhodium ion, aluminum ion sulfate radical and a polycarboxylic organic acid. Preferably, it also contains a small amount of the phosphate radical.

BACKGROUND OF THE INVENTION Electrodeposits of rhodium are beneficially used in decorative applications for jewelry and for tarnish resistant silver surfaces, in the electronics industry for printed circuitry and the like, and in general to improve the wear and/or corrosion resistance of various base metals. Commercial baths for rhodium plating, which usually comprise relatively strong acid phosphate, sulfate, or combination phosphate/sulfate solutions are generally quite satisfactory for many applications. However it is well known that high internal stresses, typically of an order of magnitude of 150,000 pounds per square inch, are frequently encountered in rhodium electrodeposits produced with such baths. Stress in the deposits tends to cause peeling, cracking, and distortion of the deposited metal, and in some instances adversely affects the substrate metal.

Attempts have been made to relieve the internal stress in rhodium plate by the addition of chloride ion or magnesium sulfate to the plating baths. Although these additives may tend to lower the internal stress of the deposit, they generally also reduce the brightness of the plate to a degree that is objectionable for many applications. As far as is known, no practical rhodium plating bath is presently available which is capable of producing a low-stress deposit that is clear, bright and substantially free of pin holes and other surface imperfections.

Accordingly, it is an object of the present invention to provide a novel rhodium electroplating bath which is capable of producing a rhodium deposit exhibiting a relatively low level of internal stress.

It is also an object to provide such a bath which is capable of producing a bright rhodium deposit that is substantially free from cloudiness, pin holes and other surface imperfections.

A further object is to provide a novel composition for producing such a bath.

Still another object of the invention is to provide a novel method for depositing rhodium from an acid bath to produce a bright electroplate that is relatively free from internal stress.

SUMMARY OF THE INVENTION It has now been found that the foregoing and related objects of the present invention can be readily attained in an aqueous acid rhodium plating bath. The bath comprises about 1.0 to 30.0 grams per liter of rhodium ion, about 0.05 to 5.0 grams per liter of aluminum ion, and about 50.0 to 400.0 grams per liter of sulfate radical, and it has a pH of about 0.1 to 2.0.

The bath also contains about 1.0 to 25.0 grams per liter of polycarboxylic organic acid which may be selected from the group consisting of azelaic acid, pimelic acid,

United States Patent O oxalic acid, malonic acid, adipic acid, glutaric acid, tartaric acid, succinic acid, citric acid, maleic acid, fumaric acid, and mixtures thereof. Most desirably, plating baths embodying the present invention will contain 2.0 to 10.0 grams per liter of rhodium ion, 0.2 to 4.0 grams per liter of aluminum ion, 3.0 to 10.0 grams per liter of the polybasic organic acid, and 100.0 to 250.0 grams per liter of sulfate radical; such a bath will preferably have a pH of at least 1.2.

Certain objects of the invention are attained by the provision, in accordance herewith, of a rhodium electroplating composition comprised of rhodium sulfate, a soluble aluminum salt, and a polybasic organic acid that is at least slightly soluble in water under plating conditions. The amounts of the various components of the composition will be selected so as to provide, upon dissolution in water, the respective amounts of the various components set forth hereinbefore.

Other objects of the invention are attained by the method of the invention, wherein a rhodium electroplating bath is employed containing the components and amounts thereof previously defined. The bath is prepared and maintained at a temperature of about 20.0 to 650 centigrade, and a workpiece having a metallic surface and an anode member are immersed therein. A voltage which is controlled to provide a current density of about 0.5 to 20.0 amperes per square foot at the surface of the workpiece is applied across the workpiece and anode member to deposit rhodium from the bath upon the metallic surface. In the preferred embodiments of the method, the bath has a pH of at least 1.2 and is maintained at a temperature of about 400 to 550 centigrade, and the current density at the workpiece surface is about 5.0 to 15.0 amperes per square foot. Most desirably, the method utilizes a preferred bath having a composition as previously set forth.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The essential components of the novel plating bath of the present invention are rhodium ion, aluminum ion, polycarboxylic organic acid and sulfate radical, and it has been found that acid plating baths containing only these active components produce bright rhodium deposits which exhibit relatively low levels of internal stress. So long as the presence of interfering ions and contaminating metals is substantially avoided, the foregoing essential components may be provided by any suitable soluble salt or other compound. However, baths free of extraneous ions and radicals may be readily be produced by dissolving in water appropriate quantities of rhodium sulfate and aluminum sulfate, with increases in acidity being effected by the addition of sulfuric acid thereto.

However, because relatively pure industrial grades of aluminum sulfate, i.e., those that are substantially free of contaminating metals, are not generally available, the aluminum constituent has conveniently been provided as the phosphate salt (i.e., aluminum orthophosphate). In fact, about 10.0 to 70.0 grams per liter of the phosphate radical, the content of which may be provided or augmented by phosphoric acid, appears to have a levelling effect in relatively thick deposits and to reduce the sensitivity of the bath to contamination; on the other hand, the optimum levels of brightness and stress relief may be achieved with the all-sulfate type of bath, and too much phosphate has been found to be detrimental. Therefore, in mixed sulfate/phosphate baths, the weight ratio of sulfate radical to phosphate radical should be at least 2.0: 1.0, with the concentration of phosphate radical not exceeding 120.0 grams per liter and preferably being in the range set forth hereinabove. Most desirably, the bath and compositions used for the production thereof will be substantially free of phosphate. To avoid any question, perhaps it should be pointed out that, as used herein, the term phosphate radical refers to the trivalent group having the formula P such as may be provided by aluminum orthophsphate and orthophosphoric acid.

The rhodium salt must be added in an amount sutficient to provide to the bath at least about 1.0 gram per liter of rhodium ion, since smaller amounts will tend to produce dull or otherwise unacceptable deposits. Conversely, the amount of the rhodium salt should not exceed that which will furnish rhodium ion to the bath in a concentration of more than 30.0 grams per liter, because greater amounts tend to be unduly expensive and wasteful, largely as a result of high drag-out losses, without aifording concomitant advantages. The best results are achieved when the amount of rhodium ion in the bath is between 2.0 and 10.0 grams per liter, and no need for amounts in excess of about 10.0 grams has been encountered.

For satisfactory deposits, it has been found that the aluminum ion should be included in the bath in an amount ranging between 0.05 and 5.0 grams per liter, and preferably the amount thereof will be from 0.2 to 4.0 grams per liter. Smaller quantities of the ion are quite ineffective, and more than about 5.0 grams per liter is uneconomical and tends to adversely affect quality. Generally, the concentration of the aluminum ion will not exceed that of the rhodium ion, and in most instances it will be considerably lower.

In addition to the essential components set forth hereinbefore, the presence of at least about 1.0, and preferably 3.0 to 10.0 grams per liter of a polycarboxylic organic acid has been found to be very beneficial. Although the organic acid may be present in an amount up to the limit of its solubility, about 25.0 grams per liter will generally be a practical maximum concentration. Exemplary of the various polycarboxylic organic acids that may be used are azelaic acid, pimelic acid, oxalic acid, malonic acid, adipic acid, glutaric acid, tartaric acid, succinic acid, citric acid, maleic acid, fumaric acid, and mixtures thereof. Although the dicarboxylic acids are especially preferred, it appears that virtually any polycarboxylic organic acid that is at least slightly soluble in the aqueous bath under plating conditions may be employed with some degree of benefit to the deposits produced thereby. The polycarboxylic acid appears to further reduce internal stress in the deposit, as well as to counteract the adverse eifects of contaminating substances, such as the small amounts of silver and zinc that may be introduced with the rhodium salt and the elemental sulfur that may be produced by sulfate oxidation.

A typical plating formulation embodying the present invention is as follows:

Water to one gallon.

Azelaic acid and pimelic acid have been used with equivalent efiicacy as the dicarboxylic organic acid in the foregoing formulation.

The pH of the bath should be maintained at a level within the range of about 0.1 to 2.0, with most satisfactory results usually being attained at pH values of at least 1.2. Lowering of the pH will generally be effected with sulfuric acid; in the event that the bath is more acidic than desired, the pH may be altered without introducing foreign ions or radicals simply by dilution with water and adding such amounts of active ingredients as 4 are necessary to provide the required concentrations of ions and radicals.

The best operating temperature for the bath is in the range of 200 to 65.0" centigrade and preferably the bath is maintained at 400 to 550 centigrade. As is commonly known, plating at unduly low temperatures tends to be inefficient, whereas temperatures that are to high tend to cause excessive consumption of bath components and also to render operation rather inconvenient and difiicult. The current density range of bright deposits will generally be from about 0.5 to 20.0 amperes per square foot, and usually the most satisfactory deposits will be produced at current densities within the range of 5.0 to 15.0 amperes per square foot. Normally, the applied voltage will be about 2 to 6 volts and will be regulated to provide an average current density at the workpiece of about 5 to 15 amperes per square foot. Whereas the actual current will depend upon the dimensions and configuration of the workpiece, values of 1 to amperes are typical.

The baths of the invention may be employed for rack and also for barrel plating, and in both cases moderate to vigorous agitation is beneficial. In accordance with conventional practice, insoluble anodes will normally be used, and they may appropriately be of platinum or platinum-surfaced construction; with such insoluble anodes, the bubbling of generated gas may be relied upon to provide the desired level of agitation in the bath. The configuration of the anode will vary depending upon the plating method and the associated equipment used; however expanded composite anodes of platinum clad tautalum or platinized titanium of the type commercially available from the American Chemical & Refining Company, *Inc. of Waterbury, .Conn., may be used to particular advantage.

With the present bath, rhodium deposits can be produced upon substantially any of the various metallic surfaces that previously have been so plated, including silver, nickel, gold, etc. The quality of the deposits that are pro duced upon copper and copper alloy surfaces is so exceptional, using the formulations defined herein that plating upon such metals represents a surprising advance in the art. The surfaces of the workpiece should be adequately cleaned in accordance with accepted practice to ensure that a high quality, adherent deposit is produced, and when the metal of the surface to be plated is soluble in the bath, a strike of insoluble metal may be applied as a precaution against contamination.

With further regard to contamination, continuous carbon treatment and filtration of the bath is recommended to remove impurities, and surfaces coming into contact with the bath should be fabricated of an inert material, such as glass, polyethylene, unplasticized polyvinyl chloride, acrylic polymers and the like. The components of the bath may be replenished on a predetermined schedule or as needed based upon bath analyses. It will, of course, be appreciated that additions of the rhodium salt will have to be made to replenish the rhodium ion, due to the use of an insoluble anode.

Although numerous techniques may be employed for the determination of internal stress in the metal deposit as is discussed by R. Weil at pages 1231-1237 of volume 12 (1970) of Plating Magazine, the plated strip method described in the examples which follow is relatively simple and affords a high level of accuracy if carefully done. The Soderberg and Graham formula for quantitively correlating the curvative of the plated strip to internal stress, which is employed in the examples, is also presented and explained in the cited publication. It may be desirable to use a second technique to check the accuracy of original values obtained, and a device such as the Brenner/ Senderofl contractometer may be employed for that purpose.

Exemplary of the eflicacy of the present invention are the following specific examples:

EXAMPLE 1 Part A An aqueous acid rhodium plating bath is prepared by admixing with water 15.0 grams per liter (as rhodium) of rhodium sulfate, 0.225 gram per liter (as aluminum) of aluminum phosphate, and 60.0 milliliters per liter of 97% sulfuric acid; the resulting solution has a pH of 0.5. Using the bath in a 267 cubic centimeter Hull cell with a. platinum anode, a gold plated brass panel is electroplated at a bath temperature of about 48 centigrade and at a current of 2 amperes for 5 minutes with about 6 volts applied, thereby producing upon the panel a rhodium deposit approximately 0.1 mil thick.

Upon inspection of the plated panel, the deposit is found to be ductile and adherent, and to be clear and fully bright over a current density range of about 0.5 to 20.0 amperes per square foot. Parenthetically, it should be noted that prior art commercial rhodium baths generally develop deposits that are undesirably dark at current densities below about 10.0 amperes per square foot.

Part B A plating bath having the composition described in Part A hereof is employed in a circular plating cell for use in an internal stress determination. A rectangular strip of copper foil 1.0 mil in thickness and measuring 0.5 inch by 5.0 inches is coated on one surface, with an acid-resistant cellulosic masking composition, and is immersed on an appropriate support backing to a depth of three inches in the bath solution. Using an inert platinum anode, a voltage is applied to produce a cathode current density at the copper foil strip of 10.0 amperes per square foot. Plating of rhodium from the solution, which is maintained at about 48 centigrade, is continued with agitation for about 15 minutes to produce a 0.13 mil deposit. The strip is removed from the beaker, carefully washed, rinsed and dried; it is then placed upon graph paper onto which its curvature is transcribed and from which the parameters necessary for using Soderberg/Graham equation are obtained. The deposit is thereby calculated to have an internal stress of about 60,000 to 61,000 pounds per square inch, compared to the typical value of about 150,000 pounds per square inch for commercial industrial rhodium plating formulations. The deposit on the strip is also found to be ductile and adherent and to have the desirable brightness and surface characteristics referred to in Part A.

EXAMPLE 2 A phosphate-free bath comparable to that prepared in accordance with Example 1 is prepared by substituting 0.4 gram per liter (as aluminum) of aluminum sulfate for the aluminum phosphate employed therein and by decreasing the sulfuric acid concentration to 50 milliliters per liter. The other components of the bath, and the plating procedures of both Parts A and B of the preceding example are employed without substantial deviation. Upon inspection of the Hull cell panel, it is found to have a rhodium deposit that is somewhat brighter than that produced with the phosphate-containing bath of Example 1. The deposit is found to be comparably ductile and adherent, and the deposit on the copper foil strip is calculated to have an internal stress of about 50,000 to 53,000 pounds per square inch.

EXAMPLE 3 For comparison with the results obtained with the baths of the two foregoing examples, a third bath is prepared wherein about 0.4 gram per liter of magnesium sulfate is employed in place of the aluminum salts employed therein, all other components in the bath and the plating procedures being substantially identical thereto. Although a relatively bright rhodium deposit is produced, it is less bright than are the deposits produced in Examples 1 and 2, and whereas the internal stress is comparable to that in the deposit produced from the phosphate-containing bath, it is found to be significantly higher than that of the deposit from the all-sulfate formulation.

EXAMPLE 4 The procedure of Example 1 is substantially repeated utilizing a formulation containing 12.0 grams per liter (as rhodium) of rhodium sulfate, 40 milliliters per liter of sulfuric acid, 0.4 gram per liter (as aluminum) of aluminum sulfate, and about 2.0 grams per liter of glutaric acid. Evaluation of the panel indicates that the presence of the dicarboxylic organic acid reduces the internal stress and improves the quality of the deposit. Substitution of oxalic, malonic, succinic, and adipic acids for the glutaric acid is found to produce comparable improvements. The substitution of azelaic or pimelic acids is also found to produce improved results, even though they are only slightly soluble in water, and maleic and fumaric acids are also found to be beneficial from the standpoint of reducing stress and producing desirable surface characteristics.

EXAMPLE 5 A bath comparable to that of Example 1 is prepared, using however suflicient aluminum phosphate and rhodium sulfate to provide only 1.4 grams per liter each of rhodium ion and aluminum ion. Although plating efficiency and stress relief are reduced, as compared with the results obtained in that previous example, a good quality deposit is obtained in which the level of internal stress is reduced from that obtained with industrial rhodium baths.

Thus, it can be seen that the present invention provides a novel rhodium electroplating bath which is capable of producing a rhodium deposit wherein the level of internal stress is relatively low. The bath also produces a bright rhodium plate that is substantially free of cloudiness, pin holes, and other surface imperfections. It generally operates with a current efficiency in excess of about to percent, and has excellent covering and throwing power compared to presently used industrial formulations. Also provided are a novel composition for producing such a bath and a novel method for producing a rhodium electrodeposit having the foregoing advantages and features.

Having thus described the invention, we claim:

1. An aqueous acid bath for the electrodeposition of rhodium comprising: about 1.0 to 30.0 grams per liter of rhodium ion, about 0.05 to 5.0 grams per liter of aluminum ion, about 50.0 to 400.0 grams per liter of sulfate radical and about 1.0 to 25.0 grams per liter of a polycarboxylic organic acid, said bath having a pH of about 0.1 to 2.0.

2. The bath of claim 1 wherein the amount, in grams per liter, of rhodium ion is 2.0 to 10.0, of aluminum ion is 0.2 to 4.0, of polycarboxylic organic acid is 3.0 to 10.0, and of sulfate radical is 100.0 to 250.0, said bath having a pH of 1.2 to 2.0.

3. The bath of claim 1 wherein said polybasic organic acid is selected from the group consisting of azelaic acid, pimelic acid, oxalic acid, malonic acid, adipic acid, glutaric acid, tartaric acid, succinic acid, citric acid, maleic acid, fumaric acid, and mixtures thereof.

4. The bath of claim 1 additionally containing about 10.0 to 70.0 grams per liter of phosphate radical, wherein the weight ratio of sulfate radical to phosphate radical is at least 20:10.

5. In a method for electrodepositing rhodium, the steps comprising:

(a) preparing an aqueous acid bath comprising about 1.0 to 30.0 grams per liter of rhodium ion, about 0.05 to 5.0 grams per liter of aluminum ion, about 1.0 to 25.0 grams per liter of a polycarboxylic organic acid and about 50.0 to 400.0 grams per liter of sulfate radical, said bath having a pH of about 0.1 to 2.0;

(b) maintaining said bath at a temperature of about 20.0 to 65.0 centigrade;

(c) immersing in said bath a workpiece having a metallic surface and an anode member; and

(d) applying a voltage across said anode member and said workpiece to deposit rhodium from said bath upon said metallic surface, said voltage providing a current density of about 0.5 to 20.0 amperes per square foot at the surface of said workpiece.

6. The method of claim wherein said bath contains 2.0 to 10.0 grams per liter of rhodium ion, 0.2 to 4.0 grams per liter of aluminum ion, 3.0 to 10.0 grams per liter of polycarboxylic organic acid, and 100.0 to 250.0 grams per liter of sulfate radical, and wherein said organic acid is selected from the group consisting of azelaic acid, pimelic acid, oxalic acid, malonic acid, adipic acid, glutaric acid, tartaric acid, succinic acid, citric acid, maleic acid, fumaric acid, and mixtures thereof.

7. The method of claim 6 wherein said bath has a pH of 1.2 to 2.0 and is maintained at a temperature of about 40.0 to .0 centigrade, and wherein said current density at said workpiece is about 5.0 to 15.0 amperes per square foot.

References Cited UNITED STATES PATENTS OTHER REFERENCES F. H. Reid, Metalloberflache, vol. 15, pp. 3337 (1961).

GERALD L. KAPLAN, Primary Examiner 

